X-ray tubes used in medical diagnostic imaging are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the x-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The X-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
Such an arrangement is typical of rotating X-ray tubes and has remained relatively unchanged in concept of operation since its introduction. Unfortunately, this type of motor arrangement with the combined stator and rotor is very inefficient. For example, in low speed steady state operation, where the rotor is driven with a frequency of 3600 rotations per minute (RPM), the peak motor efficiency is only approximately 8% and the average motor efficiency is only approximately 4.5%. In high speed steady state operation, where the rotor is driven with a frequency of 10,800 RPM, the peak motor efficiency is approximately 24% and the average motor efficiency is approximately 12%. The poor efficiency of the current motor design results in low torque delivered to the rotor assembly and a large amount of heat (typically 300-400 Watts in the run mode of operation) delivered to the tube housing environment.
The low efficiency of the existing design results from the need to employ a significant air gap between the rotor assembly and stator due to the differences in potential. As the stator operates at or near ground potential and the anode may be raised to 75,000 VDC positive with respect to ground, a large air gap on the order of 0.400 inches or greater is needed to maintain stable, discharge free operation in the current design techniques.
Several attempts have been made to improve the efficiency of the stator through other design techniques, such as increasing the length of the stator core and varying the material in the stator core. Unfortunately, such attempts have resulted in, at best, only incremental improvements in delivered torque to the rotor. For instance, a 40% increase in stator core length yields only a 25% improvement in delivered torque to the rotor assembly during steady state run operation. Also, it has been found that variations in the core material have little to no significant effect.
It would be desirable then to have a design which improves the efficiency of the stator and rotor assembly.